Fungal community dynamics and C mineralization in coarse woody debris

Fungi are primary agents of coarse woody debris (CWD) decay in forests, playing an essential role in nutrient cycling and carbon storage. Characterizing fungal communities within CWD will promote further understanding of the fungal controls on CWD decomposition. We compared fungal community assemblages using both alpha and beta diversity metrics, CO2 production, and physical and chemical properties of CWD across 3 tree species (Populus tremuloides, Picea mariana, and Pinus banksiana), 5 decay classes, and 2 stand development stages, differing in time-since-stand replacing disturbance (i.e., young/self-thinning and mature/steady-state) in Ontario's boreal forest region. In total, we sampled 180 individual CWD logs from 6 independent stands, with 3 replicates per each species x decay class combination at each site. We found that fungal community structure significantly differed across tree species, decay stage, and stand age. Higher proportions of white rot fungi were found in P. tremuloides CWD, whereas higher proportions of brown rot fungi were found in P. banksiana and P. mariana CWD. Proportions of specialized wood decay fungi (white rot, brown rot, and soft rot) increased with decay stage, and were higher in CWD located in mature forest stands. Fungal diversity was highest in decay class 4 CWD. We found that Mn and K concentrations, total carbon, C/N ratio, carbon mineralization (mg CO2 g dry CWD-1 d-1), and moisture content were important predictors of fungal composition across CWD species and/or decay stage, though it is unknown how CWD chemistry influences fungal species composition and vice versa. Carbon mineralization (mg CO2 g dry CWD-1 d-1) was highest in P. tremuloides CWD and increased with decay stage, perhaps facilitated by increased N concentrations. This study highlights the unique contributions of different tree species, age classes, and decay classes to fungal biodiversity, and suggests that forest management guidelines that consider both deadwood quantity and quality will provide a broader array of habitat characteristics. This, in turn, will support a broader range of fungal species and communities through post-disturbance stand development, thereby conserving biodiversity and keystone species over the longer-term in our managed forest systems.